The invention relates to a fast neutron nuclear reactor of integrated type, i.e. in which the complete primary circuit of the reactor is contained in a vertically axed vessel, called the main vessel and whose upper part is sealed by a slab. More specifically the invention relates to a reactor in which the "hot" liquid metal (generally sodium) leaving the reactor core is separated from the "cold" liquid metal leaving the exchangers by an internal cylindrical vessel.
In fast neutron reactors, the liquid metal is heated during its passage in the core as a result of the fission reaction taking place within the latter. The heat stored in the liquid metal contained in the primary circuit is then transferred to the liquid metal (generally sodium) contained in a secondary circuit by intermediate heat exchangers. The primary liquid metal is then taken up again by the primary pumps to be reinjected into the core.
In reactors of the integrated type, the exchangers and pumps are suspended on the slab sealing the main vessel of the reactor. Moreover, in reactors with an internal cylindrical vessel, the exchangers and pumps are located in the annular space defined between the inner vessel and the main vessel. Throughout the remainder of the text the zone defined within the inner vessel and containing the liquid metal leaving the reactor core is called the "hot" collector and the zone containing the liquid metal leaving the exchangers and which has not yet entered the reactor core is called the "cold" collector.
Compared with fast neutron reactors in which the cold collector is separated from the hot collector by a stepped inner vessel traversing the exchangers and the pumps, reactors with a cylindrical inner vessel have the advantage of reduced overall dimensions with respect to the diameter of the reactor block.
However, in fast neutron reactors with an inner cylindrical vessel there are a certain number of problems related to the connection between the inner vessel and the exchangers. Thus, the pipes and ferrules bringing about this connection support both the mechanical loads resulting from their own weight, the hydrostatic pressure forces resulting from the tight connection with has to be provided between these structures and the wall of the exchanger in order to effectively insulate the hot collector from the cold collector, the thermal stresses due to the temperature differences of the liquid metal between the collectors and the stresses linked with the hydraulic function played by these structues for the supply of the exchangers. It is obvious that the accumulation of all these stresses on one and the same structure makes its construction extremely difficult, especially in view of the fact that there must be no risk of it failing.